Referring to FIG. 1, a conventional machine tool includes a bit-driving unit 10 and a bit-changing unit. The bit-changing unit is used to carry and change holders 24 for holding bits so that a selected one of the bits is engaged with the bit-driving unit 10. The conventional machine tool includes a beam 2 formed thereon, a supporting element 12 connected to the beam 2 and a pinion 11 provided thereon. The bit-changing unit includes a rocker 25 connected to the supporting element 12 with a pivot 14 and a disc 20 rotationally connected to the rocker 25. A gear 23 is engaged with the pinion 11 and a reduction device 22 is provided between the disc 20 and the gear 23 so that the disc 20 is operated at a low speed as the pinion 11 is operated at a high speed. A roller 18 is supported on the rocker 25 and movable against a cam 13 attached to the bit-driving unit 10. The cam 13 includes an upper rectilinear section 15, a lower rectilinear section 17 and an arched section 16 formed below the upper rectilinear section 15 and the lower rectilinear section 17. A pulling element 26 such as a spring is provided between the rocker 25 and the bit-driving unit 10. In use, one of the holders 24 is engaged with the bit-driving unit 10, and the bit-driving unit 10 is located in a lower position where the roller 18 is in contact with the upper rectilinear section 15 of the cam 13. To change the holder 24 with another holder 24, the bit-driving unit 10 is lifted to a middle portion where the roller 18 is in contact with an upper end of the arched section 16 of the cam 13. The pulling element 26 pivots the rocker 25 about the pivot 14 so that one of the clips 21 is moved towards the holder 24. Then, the bit-driving unit 10 is stopped. The pulling element 26 further pivots the rocker 25 so that the clip 21 is engaged with the holder 24, i.e., the clip 21 enters a V-shaped recess defined in the holder 24. The bit-driving unit 10 is lifted to an upper position so that the holder 24 is disengaged from the bit-driving unit 10.
Referring to FIG. 2, there is shown another conventional machine tool including a bit-driving unit 10 and a bit-changing unit. The bit-changing unit includes a rocker 25 pivotally connected to the machine tool with a pivot 14. A disc 20 is rotationally connected to the rocker 25. Clips 21 are attached to the disc 20. The clips 21 are used to clip holders 24. A pulling element 26 is provided between the rocker 25 and the machine tool. A roller 31 is attached to the rocker 25. A lower track 30 is attached to the bit-driving unit 10. The roller 31 is movable against the lower track 30. An end of the lever 32 is connected to the bit-driving unit 10. A roller 33 is attached to another end of the lever 32. An upper track 34 is attached to the machine tool. The upper track 34 includes a groove for receiving the roller 33 so that the lever 32 is pivoted when the roller 33 is rolled in the groove of the upper track 34 as the bit-driving unit 10 is moved vertically. A threaded bolt 35 is connected to the lever 32. A nut 36 is engaged with the threaded bolt 36. The nut 36 can be abutted against an element 37 to prevent undesired rotation of an element 38 and the disc 20.
In the conventional bit-changing units, a selected one of the clips 21 is movable along an arched path while a related one of the holders 24 is movable along a vertical path. The arched path intersects with the vertical path at only a point. The selected clip 21 will hit and damage the related holder 24 if the latter is kept in position while the former is moved to the latter. To avoid this problem, the related holder 24 is moved slightly when the selected clip 21 is moved thereto. This requires precise operation of a complicated mechanism.
The present invention is therefore intended to obviate or at least alleviate the problems encountered in prior art.